Method of manufacturing high strength and high ductility titanium alloy

ABSTRACT

Disclosed is a method of manufacturing a high strength and high ductility titanium alloy. The method comprises: providing a titanium alloy having a martensite structure; and partially dynamically spheroidizing a microstructure through a thermal and mechanical treatment of the titanium alloy having the martensite structure. According to the present invention, a titanium alloy having a partially dynamically spheroidized microstructure can be manufactured to have excellent yield strength (YS) and uniform elongation (U.EL). A microstructure having lamellar structures is controlled to a microstructure where fine equiaxed structures and lamellar structures are simultaneously present by regulating a rolling direction and a deformation amount. According to the present invention, a titanium alloy can be manufactured to have an improved product (YS×U.EL) of yield strength and uniform elongation as compared with conventional heat treatment.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2009-0130762 filed in the Korean Intellectual Property Office on Dec. 24, 2009, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a titanium alloy, and more particularly, to a method of manufacturing a titanium alloy having a microstructure where fine equiaxed structures and lamellar structures are mixed due to partial dynamic spheroidization.

(b) Description of the Related Art

Yield strength and uniform elongation are very important mechanical characteristics in the case of a metallic material, such as a titanium alloy, used in an extremely harsh environment. When strength higher than yield strength of a titanium alloy mainly used for a structural material is applied to the titanium alloy from the outside, the material is permanently deformed. Thus, it is important to obtain high yield strength the titanium ally.

Further, when deformation in a material is higher than a uniform elongation, necking is generated at a vulnerable portion of the material, causing the material to fracture. Thus, it is also inevitably necessary to obtain a high uniform elongation in order to improve reliability of a structural material.

However, when a titanium material is manufactured through a conventional heat treatment, yield strength and uniform elongation of the material tend to be inversely proportional to each other. Thus, various methods for overcoming the problem have been suggested. In recent years, Korean Patent Application Laid-Open No. 2009-0069647 (2009.07.01) discloses a method of improving yield strength and ductility of an alloy obtained by adding niobium to titanium as compared with pure titanium.

However, this method relates to an alloying before a thermal/mechanical treatment is performed, and the category of the method is different from the present method by which strength and ductility of an alloy are increased through a thermal/mechanical treatment after the alloying is performed.

Meanwhile, Korean Patent Application No. 10-2009-0083931 (2009.09.07) which was filed by the present applicant discloses a method of cross rolling a titanium alloy having fine lamellar structures in a worm region to make ultrafine crystal grains in the titanium ally.

In more detail, a process parameter is optimized by inducing an initial microstructure to martensite having fine lamellar structures and observing an influence by a deformation amount, a deformation rate, and a deformation temperature on a change in microstructures, making it possible to manufacture a titanium alloy having a equiaxed structure of nano-sized crystal grains at a low deformation amount.

However, even though the method remarkably increases yield strength, uniform elongation severely decreases as compared with a conventional heat treatment method. Thus, a product of yield strength and uniform elongation is not largely improved as compared with a conventional microstructure but becomes smaller.

Accordingly, a technology for improving a balance between yield strength and uniform elongation through a thermal and mechanical treatment is necessary to increase reliability and applications of a titanium alloy.

The present invention has been made in an effort to provide a method of manufacturing a titanium alloy where equiaxed microstructures and lamellar structures are mixed through partial dynamical spheroidization of microstructures by thermally and mechanically treating a titanium alloy to maintain a balance between yield strength and uniform elongation.

An exemplary embodiment of the present invention provides a method of manufacturing a high strength and high ductility titanium alloy, the method including: providing a titanium alloy having a martensite structure; and partially dynamically spheroidizing a microstructure through a thermal and mechanical treatment of the titanium alloy having the martensite structure.

The microstructure of the provided titanium alloy may include a lamellar martensite structure.

During the thermal and mechanical treatment, the titanium alloy may be rolled at a deformation temperature of 775° C. to 875° C., a deformation rate of 0.07 s⁻¹ to 0.13 s⁻¹, and a deformation amount of −0.2 to −1.6.

During the thermal and mechanical treatment, the titanium alloy may be rolled at a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −0.2 to −1.6.

The rolling may be uni-directional rolling.

Fine equiaxed structures and lamellar structures may be simultaneously present in the microstructure of the titanium alloy through the partial dynamic spheroidization.

According to the exemplary embodiment of the present invention, a titanium alloy having excellent yield strength and uniform elongation can be produced, which improves reliability in an in-use environment and enlarges application ranges.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an optical microscopic picture of a Ti-6Al-4V alloy having initial equiaxed structures.

FIG. 2 is a picture showing a martensite structure obtained by water cooling a titanium alloy after maintaining the titanium alloy at 1,040° C. for an hour.

FIG. 3 is a picture showing coarse lamellar structures obtained by maintaining a titanium alloy at 1,040° C. for 4 hours, air cooling the titanium alloy, maintaining the titanium alloy at 730° C. for 4 hours, and then air cooling the titanium alloy.

FIG. 4 is a picture showing dual structures obtained by maintaining a titanium alloy at 950° C. for 4 hours, water cooling the titanium alloy, maintaining the titanium alloy at 540° C. for 6 hours, and then air cooling the titanium alloy.

FIG. 5 is an electron backscattered diffraction picture of a microstructure when a Ti-6Al-4V alloy having martensite structures is uni-directionally rolled at a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −0.2.

FIG. 6 is an electron backscattered diffraction picture of a microstructure when a Ti-6Al-4V alloy having martensite structures is rolled in one direction at a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −0.8.

FIG. 7 is an electron backscattered diffraction picture of a microstructure when a Ti-6Al-4V alloy having martensite structures is rolled in one direction at a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −1.2.

FIG. 8 is an electron backscattered diffraction picture of a microstructure when a Ti-6Al-4V alloy having martensite structures is rolled in one direction at a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −1.6.

FIG. 9 is an electron backscattered diffraction picture of a microstructure when a Ti-6Al-4V alloy having martensite structures is cross rolled, in which case a process condition has a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −1.6.

FIGS. 10, 11 and 12 respectively show room temperature tensile test results of titanium alloys having microstructures, in which FIG. 10 represents average yield strengths, FIG. 11 represents average uniform elongations, and FIG. 12 represents products of average yield strengths and average uniform elongations.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention will be described in detail.

After an initial microstructure is induced to martensite having fine lamellar structures to obtain partially dynamically spheroidized microstructures (that is, a microstructure where fine equiaxed structures and lamellar structures are simultaneously present), an influence by a rolling direction, a deformation amount, a deformation rate, and a deformation temperature on a change in microstructure was observed.

FIGS. 1 to 4 are pictures observed by using an optical microscope, and show representative microstructures which can be obtained through a conventional heat treatment. FIG. 1 shows an initial microstructure of a Ti-6Al-4V alloy which has equiaxed structures having a crystal grain size of 10 μm.

FIG. 2 shows a martensite structure having fine lamellar structures obtained by maintaining the microstructure of FIG. 1 at 1,040° C. which is higher than a beta (β) transformation temperature (˜1,000° C.) for one hour and water cooling the microstructure.

FIG. 3 shows lamellar structures having coarse lamellar structures obtained by maintaining the microstructure of FIG. 1 at 1,040° C. for 4 hours, air cooling the microstructure, maintaining the microstructure at 730° C. for 4 hours, and then air cooling the microstructure.

FIG. 4 shows a dual structure with coarse equiaxed structures and lamellar structures obtained by maintaining the microstructure of FIG. 1 at 950° C. for 4 hours, water cooling the microstructure, maintaining the microstructure at 540° C. for 6 hours, and then air cooling the microstructure.

FIGS. 5 to 8 are inverse pole figures (IPFs) obtained by uni-directionally rolling a Ti-6Al-4V alloy having the martensite structures of FIG. 2 while changing a process condition and observing the TI-6Al-4V alloy with an electron backscattered diffraction (EBSD) apparatus.

The process condition of FIG. 5 has a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −0.4.

The process condition of FIG. 6 has a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −0.8.

The process condition of FIG. 7 has a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −1.2.

The process condition of FIG. 8 has a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −1.6.

As shown in FIGS. 5 to 8, as the deformation amount increases when uni-directional rolling is performed, a ratio of fine equiaxed structures formed by segmenting the martensite structure of FIG. 2 increases, but fine equiaxed structures and lamellar structures (portions indicated by a red color) are simultaneously present in the microstructures of FIGS. 5 to 8.

The differences between the structure of FIG. 4 and the microstructures of FIGS. 5 to 8 lie in that the coarse equiaxed structures and the lamellar structures forming a colony are mixed in FIG. 4, whereas fine equiaxed structures and lamellar structures which do not form a colony are mixed in FIGS. 5 to 8. Meanwhile, a ratio of the fine equiaxed structures increases as a deformation amount increases because subgrains produced in the lamellar structures are effectively changed into crystal grains having a high angle grain boundary.

As a result, the microstructures and the process conditions of FIGS. 5 to 8 are the essence of the present invention.

FIG. 9 is an inverse pole figure (IPF) obtained by cross rolling a Ti-6Al-4V alloy having martensite structures of FIG. 2 at a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −1.6, and observing the Ti-6Al-4V alloy with an electron backscattered diffraction apparatus.

FIG. 9 shows fine equiaxed structures obtained through complete dynamic spheroidization. When comparing FIG. 9 with FIG. 8, the process conditions such as a deformation temperature, a deformation rate, and a deformation amount are the same as each other, but the rolling directions are different from each other.

That is, FIG. 8 is obtained by uni-directional rolling, and FIG. 9 is obtained by cross rolling. Unlike uni-directional rolling, in the case of cross rolling, lamellar structures which have not been segmented in an odd-numbered rolling step are effectively segmented in an even-numbered step. As a result, a completely dynamically spheroidized microstructure is obtained, but it is required to avoid this condition to manufacture a partially dynamically spheroidized titanium alloy.

Meanwhile, the mechanical characteristics of all the above-mentioned microstructures at a room temperature will be described. To this end, after a specimen having a gauge length of 25 mm is extracted at three directions of 0°, 45°, and 90° with respect to a rolling direction and the specimen is mounted to an extensometer, three tensile experiments were performed with respect to the directions by using INSTRON 8801.

That is, a total of nine experiments were repeated on the microstructures. FIGS. 10 to 12 represent average values of room temperature tensile test results, and Table 1 represents the microstructures and heat treatment between Comparative Examples and Examples.

TABLE 1 No. Microstructure Heat Treatment Comparative Initial equiaxed 930° C., 6 hours, furnace cooling Example 1 structure Comparative Martensite 1,040° C., 1 hour, water cooling Example 2 structure Comparative Lamellar 1,040° C., 4 hours, air cooling + Example 3 structure 730° C., 4 hours, air cooling Comparative Dual structure 950° C., 4 hours, water cooling + Example 4 540° C., 6 hours, air cooling Example 1 Partially 1,040° C., 1 hour, water cooling + dynamically 800° C., 0.1 s⁻¹, uni-axial rolling spheroidized (deformation amount: −0.4) structure Example 2 Partially 1,040° C., 1 hour, water cooling + dynamically 800° C., 0.1 s⁻¹, uni-axial rolling spheroidized (deformation amount: −0.8) structure Example 3 Partially 1,040° C., 1 hour, water cooling + dynamically 800° C., 0.1 s⁻¹, uni-axial rolling spheroidized (deformation amount: −1.2) structure Example 4 Partially 1,040° C., 1 hour, water cooling + dynamically 800° C., 0.1 s⁻¹, uni-axial rolling spheroidized (deformation amount: −1.6) structure Comparative Completely 1,040° C., 1 hour, water cooling + Example 5 dynamically 800° C., 0.1 s⁻¹, cross rolling spheroidized (deformation amount: −1.6) structure

FIG. 10 represents average yield strengths of microstructures, FIG. 11 represents average uniform elongations of the microstructures, and FIG. 12 represents products of average yield strengths and average uniform elongations of the microstructures.

When conventional heat treatment of Comparative Examples 2, 3, 4, and 5 are compared with an initial microstructure of Comparative Example 1, average yield strength increased, but average uniform elongation decreased.

On the contrary, in Example 1 manufactured according to the method of the present invention, average yield strength was similar, but average uniform elongation increased as compared with the initial microstructure of Comparative Example 1. Further, in Examples 2 and 3, both average yield strengths and average uniform elongations increased as compared with the initial microstructure of Comparative Example 1. Furthermore, in Example 4, average yield strength increased and average uniform elongation was similar as compared with the initial microstructure of Comparative Example 1.

Consequently, Examples 1, 2, 3 and 4 manufactured by the method of the present invention showed a product of average yield strength and average uniform elongation which has been enhanced by 25 to 100% or more as compared with the other heat treatment methods.

While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A method of manufacturing a high strength and high-ductility titanium alloy, the method comprising: providing a titanium alloy having a martensite structure; and partially dynamically spheroidizing a microstructure through a thermal and mechanical treatment of the titanium alloy having the martensite structure.
 2. The method of claim 1, wherein: the microstructure of the provided titanium alloy includes a lamellar martensite structure.
 3. The method of claim 1, wherein: during the thermal and mechanical treatment, the titanium alloy is rolled at a deformation temperature of 775° C. to 875° C., a deformation rate of 0.07 s⁻¹ to 0.13 s⁻¹, and a deformation amount of −0.2 to −1.6.
 4. The method of claim 3, wherein: during the thermal and mechanical treatment, the titanium alloy is rolled at a deformation temperature of 800° C., a deformation rate of 0.1 s⁻¹, and a deformation amount of −0.2 to −1.6.
 5. The method of claim 1, wherein: the rolling is uni-directional rolling.
 6. The method of claim 5, wherein: fine equiaxed structures and lamellar structures are simultaneously present in the microstructure of the titanium alloy through the partial dynamic spheroidization.
 7. The method of claim 2, wherein: the rolling is uni-directional rolling.
 8. The method of claim 7, wherein: fine equiaxed structures and lamellar structures are simultaneously present in the microstructure of the titanium alloy through the partial dynamic spheroidization.
 9. The method of claim 3, wherein: the rolling is uni-directional rolling.
 10. The method of claim 9, wherein: fine equiaxed structures and lamellar structures are simultaneously present in the microstructure of the titanium alloy through the partial dynamic spheroidization.
 11. The method of claim 4, wherein: the rolling is uni-directional rolling.
 12. The method of claim 11, wherein: fine equiaxed structures and lamellar structures are simultaneously present in the microstructure of the titanium alloy through the partial dynamic spheroidization. 